<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<body>
    <script>
        var score = prompt('请输入分数');
        if (!isNaN(score)) {
            if (score >= 90) {
                alert('A');
            } else if (score >= 80) {
                alert('B');
            } else if (score >= 70) {
                alert('C');
            } else if (score >= 60) {
                alert('D');
            } else {
                alert('E');
            }
        } else {
            alert('请输入数字');
        }
    </script>
</body>
</html>